Schist

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Schist

The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar.

The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen by the naked eye. Schist is characteristically foliated, meaning the individual mineral grains split off easily into flakes or slabs. The characteristic flaky texture of schist gives rise to the adjective "schistose".

Manhattan schist, from Southeastern New York

Most schists have in all probability been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as basalts and tuffs. Most schists are mica schists, but graphite and chlorite schists are also common.

Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist, glaucophane schist, etc.

The word schist is derived from the Greek meaning "to split", which is in reference to the ease with which schists can be split along the plane in which the platy minerals lie.

It is the most abundant rock in the state of New Hampshire.

Schists are frequently used in building houses or walls, as many are quite durable and strong. However it should be noted that many foundation problems with buildings both large and small are due to the Schist decaying or even the failure of the mortar. This in turn lets water into the joints thus weakening the schist further.

Most of the building foundations built in the 1920's and 30's within the NYC area used schist. Decorative rock walls on houses in the NYC area also used a schist called "Yonkers Stone" which is no longer available. This schist was particularly hard and color fairly consistant.